A thermal processing chamber as used herein refers to a device that rapidly heats objects such as semiconductor wafers. Such devices typically include a substrate holder for holding a semiconductor wafer and a light source that emits light energy for heating the wafer. During heat treatment, the semiconductor wafers are heated under controlled conditions according to a preset temperature regime. For monitoring the temperature of the semiconductor wafer during heat treatment, thermal processing chambers also typically include radiation sensing devices, such as pyrometers, that sense the radiation being emitted by the semiconductor wafer at a selected wavelength. By sensing the thermal radiation being emitted by the wafer, the temperature of the wafer can be calculated with reasonable accuracy.
Many semiconductor heating processes require a wafer to be heated to high temperatures so that various chemical and physical transformations can take place. For instance, semiconductor wafers are typically heated to temperatures of from about 400.degree. C. to about 1200.degree. C., for times which are typically less than a few minutes. During heat treating, one main goal is to heat the wafers as uniformly as possible. During some applications, a gas or gases can be circulated through the thermal processing chamber as the semiconductor wafer is being heat treated. The gas or gases can contain or can be converted to a gaseous reactant which is designed to react with the surface of the semiconductor wafer in order to form a film or coating on the wafer. For instance, the film or coating can be applied to the wafer in order to produce a semiconductor device having desired properties.
For example, in one particular application, a gas containing dinitrogen oxide is fed to a thermal processing chamber containing a semiconductor device in order to form an oxide coating on the surface of the semiconductor device. More particularly, as the gas is fed to the thermal processing chamber, the dinitrogen oxide increases in temperature due to exposure to the heat source in communication with the chamber and due to contact with the wafer itself. When the dinitrogen oxide reaches a certain temperature, the compound disassociates forming nitric oxide which reacts with silicon contained within the wafer to form silicon oxynitride.
In the past, various problems have been experienced in attempting to form films and coatings on semiconductor wafers according to the above-described process. For instance, since the gas or gases entering the thermal processing chamber must be heated before a reaction will occur with a semiconductor device, the process proceeds at a relatively slow rate which limits the speed at which the semiconductor devices can be fabricated.
Another problem that has been experienced in the past is the ability to form a coating on the semiconductor wafer that has a uniform thickness. For example, since the gas must be heated prior to reaction with the wafer, more reaction occurs at the far end of the wafer than at the end of the wafer that first contacts the gas. In some systems, the wafer is rotated during the process in order to promote a more uniform reaction with the gas over the surface of the wafer. Rotating the wafer, however, does not completely cure the above deficiencies and thus problems are still experienced in forming a coating or film having a uniform and controlled thickness.
Another problem experienced in thermal processing chambers in which a gas is circulated is the cooling effect the gas has on the semiconductor wafer. In particular, as the wafer is exposed to the flow of gas, increased convective cooling may occur over the surface of the wafer and along its edges, particularly where the gas first contacts the wafer. Ultimately, these energy losses can create different temperature zones within the wafer, which adversely impacts upon the heat treatment process and upon the quality of the final product.
For illustrative purposes, referring to FIG. 1, a schematic diagram of a conventional thermal processing chamber generally (10) is shown. Thermal processing chamber (10) includes a plurality of light sources (12) for heating a semiconductor wafer (14). As shown, a gas to be reacted with wafer (14) is fed through a gas inlet and over the surface of the wafer.
The gas is introduced into thermal processing chamber (10) at a temperature T.sub.i which is below the temperature at which the gas will react with the wafer. As the gas flows over the wafer, the gas increases in temperature until it reaches a critical temperature T.sub.c necessary for reaction with the wafer to occur. For instance, if the gas being circulated through the chamber contains dinitrogen oxide, the critical temperature is the temperature at which the dinitrogen oxide disassociates into nitric oxide.
As shown in the figure, typically the gas is not heated to its critical temperature until the gas has already traveled a substantial distance over the surface of the wafer. Thus, most of the reaction occurs at the far end of the wafer resulting in a coating having an uneven thickness. Also, as described above, the gas circulating through the chamber can cool the wafer and prevent the wafer from being heated uniformly.
In view of the above deficiencies and drawbacks of prior art constructions, currently, a need exists for a method of uniformly coating semiconductor wafers using a gaseous reactant. A need further exists for a method of coating semiconductor wafers with a gaseous reactant without adversely affecting the heat treating process.